1. Field of the Invention
The present invention relates to a vehicle AC generator, and particularly relates to improvement in a frame structure of the vehicle AC generator.
2. Description of Related Art
There is a strong demand that vehicle AC generators should be compact in size and light in weight, and have high output power and high efficiency. To address such a demand, the vehicle AC generators need to have enhanced air cooling performance. Accordingly, a typical conventional vehicle AC generator is configured as follows. The rear frame thereof includes a peripheral wall section and an end wall section. The end wall section has a centrifugal blade-facing portion facing axial front ends of centrifugal blades of a cooling fan, and a coil end-facing portion facing a coil end of a stator coil. The coil end-facing portion, which extends from the centrifugal blade-facing portion toward the peripheral wall section, is bent at a predetermined curvature radius. The reason why the coil end-facing portion is bent to the side of the coil end of the stator coil is to satisfactorily cool the coil end by deflecting the cooling air, which is being blown radially outward along an inner end surface of the end wall section by the centrifugal blades (cooling fan) mounted to a rotor end surface, toward the coil end of the stator coil as much as possible.
It is known to form cooling air passages in the coil end of a stator coil to improve the cooling performance, as disclosed in Japanese Patent Gazette No. 2997288 which describes a vehicle AC generator employing a stator coil of the segment-sequential-connection type.
It is also known to increase the size of a cooling air discharge window formed in an outer periphery of a frame of a vehicle AC generator in order to let the cooling air flow smoothly from the discharge window, as disclosed in Journal of Nippon Denso Technical Disclosure published Mar. 15, 1987.
It is also known to provide a fan guide facing a cooling fan with a projection at a position radially outwardly of the cooling fan in order to block the cooling air backflowing through a clearance between the cooling fan and the fan guide, to thereby improve the cooling efficiency, as disclosed in Journal of Japan Institute of Invention and Innovation published Dec. 20, 1986.
However, the stator coil cooling capacity of the structure disclosed in Japanese Patent Gazette No. 2997288 is insufficient to meet the increasing demand that vehicle AC generators should be compact in size and light in weight, and have high output power and high efficiency. In addition, to attain high perceived quality in quietness of a vehicle, it becomes necessary to redue the noise of the cooling fan. Accordingly, it has been proposed to increase the size of the cooling fan, or the number of the fan blades, because increasing the cooling air volume is effective in increasing the stator coil cooling capacity. However, increasing the cooling air volume leads to upsizing of the vehicle AC generator, and increase of the fan driving power. This also derives the problem of large wind noise.
Journal of Nippon Denso Technical Disclosure mentioned above discloses a structure in which a step portion is formed in a rear frame end wall section so that the height of the cooling air discharge window can be increased to smoothly discharge the cooling air therethrough without changing the longitudinal length of the generator. However, this structure does not improve the stator coil cooling capacity. This structure is intended to improve a rectifier device cooling capacity by increasing the cooling air volume, however, this cause the problem of large wind noise as in the above case.
Journal of Japan Institute of Invention and Innovation mentioned above discloses forming a ring-shaped projection in the fan guide to prevent the cooling air blown by the cooling fan from backflowing, to thereby improve the cooling efficiency. However, this structure does not have means for improving the stator coil cooling capacity. In addition, in this structure, the cooling fan may be deformed and interfere with other members when the rotor of the generator is driven at high speed (20,000 rpm, for example.)